The term “battery” originally meant a plurality of galvanic cells connected in series. Nowadays, however, individual galvanic cells are often also referred to as a battery. When a galvanic cell is being discharged, a chemical reaction delivering energy takes place, which consists of two subreactions electrically coupled to one another, but spatially separated from one another. At the negative electrode, electrons are released in an oxidation process, resulting in a flow of electrons via an external load to the positive electrode, by which a corresponding quantity of electrons is received. At the positive electrode, a reduction process thus takes place. Simultaneously, a flow of ions corresponding to the electrode reaction takes place inside the cell. This flow of ions is ensured by an ion-conducting electrolyte. In secondary cells and batteries, this discharge reaction is reversible, that to say the conversion of chemical energy taking place during the discharge can be reversed into electrical energy.
Among known secondary cells and batteries, relatively high energy densities are achieved in particular by lithium ion batteries. In many cases, lithium ion batteries contain a cell stack consisting of a plurality of individual cells. Wound cells (coils) are also frequently used. The cells in a lithium ion battery are usually a composite of electrode foils and separator sheets with the sequence positive electrode/separator/negative electrode. Often, such individual cells are produced as so-called “dual” cells with the possible sequences negative electrode/separator/positive electrode/separator/negative electrode or positive electrode/separator/negative electrode/separator/positive electrode. The electrodes in that case conventionally comprise metal current collectors, which are usually in the form of flat structures. The case of positive electrode usually involves meshes or sheets of aluminum, for example, made of expanded aluminum metal or a perforated aluminum foil. On the side of the negative electrode, meshes or foils of copper are usually employed.
In general, the described cells for lithium ion batteries are produced in a multistage method. It is conventional for the aforementioned electrode foils to be produced in a first step, and for these then to be combined with one or more separator sheets to form the aforementioned electrode/separator composites. The electrodes and separators are usually bonded to one another in a lamination step.
To produce the electrodes, a flat layer of a paste-like material (“paste” for short), which contains electrochemically active particles, is usually applied onto a suitable collector and subsequently dried. The paste is preferably applied onto both sides of the collector. In terms of production technology, this is usually carried out by providing the collectors as quasi-endless strips, which subsequently pass through a coating unit in which a deposited coating of the collector, interrupted at defined distances in the direction of passage, is applied by intermittent coating. The collector strip emerging from the coating unit correspondingly has coated and uncoated regions alternating in the direction of passage. The collector strip may subsequently be divided up by cutting the strip in the uncoated regions.
The pastes are, for example, applied onto the collectors with a doctor blade, although wide-slot nozzles (slot-die nozzles), which are outstandingly suitable for high-throughput applications, may also be used. They have a slot-shaped delivery opening for the paste, which is supplied to them from a paste reservoir via a transport channel. For the intermittent coating, the paste flow in the direction of the delivery opening is regulated by a control means which blocks it at regular time intervals. In particular, in the case of pastes having high proportions of elastic, however, this procedure entails disadvantages. Such pastes relax, by viscous flow processes, after interruption of the paste supply in the direction of the slot-shaped delivery opening, where a drop can form, the effect of which is that an edge with an undesired thickness is formed in the next coating interval. The drop may possibly also touch the collector strip passing through and contaminate uncoated regions of the strip.
US 2004/0062866 A1 discloses a coating apparatus by which paint is applied discontinuously onto a flat base material. The paint is pumped from a reservoir into a nozzle head in which there is a collection container to which a reduced pressure can be applied by a plunger system. When the coating is interrupted, a reduced pressure can be applied to the collection container, specifically by the aforementioned plunger system. Inadvertent paint delivery can thereby be counteracted. However, there is no mention of the processing of pastes for electrodes in US 2004/0062866 A1.
It could therefore be helpful to provide a device and a method for intermittent coating of a moving surface with a paste which contains electrochemically active particles, with which the described problems relating to the formation of drops during the coating process can be avoided.